Lights of the type described above are known, in which the electronic control circuit is configured to control the turning on and/or off of all LEDs simultaneously, producing a luminous effect very similar to that obtained by turning on or off a single-source traditional light, i.e. provided with only one filament lamp.
FIG. 1 shows a lighting device for lights of a known type, indicated as a whole with 1, comprising an array of light emitting sources 2, and an electronic control circuit 3 configured so as to simultaneously turn on/off, the array of light emitting sources 2.
The array of light emitting source 2 comprises a series of lighting branches 4, which are connected in parallel to one another between a first 5 and a second 6 common nodes and each comprises a series of LEDs 7 arranged in series one after the other between the common node 5 and a sensing node 8; a resistor 9 having a first terminal connected to the LED 7 through the sensing node 8, and a diode 10 having the anode connected to the second terminal of the resistor 9 and the cathode connected to the common node 6.
As for the electronic control circuit 3, it comprises a power stage 11 and a driving stage 12.
In particular, the power stage 11 comprises a power unit 13 presenting a first and a second output terminal, which are placed at a voltage VA and respectively at a reference voltage VR e.g. corresponding to a ground voltage VGND, are flowed through by a primary supplying current I1, and are connected to common nodes 5 and 6 of the array of light emitting source 2 via a power supply line 14 and a power supply line 15.
The power stage 11 also comprises a protection diode 16 provided along the power supply line 14 to protect the LEDs 7 from accidental conditions of reverse polarity of the voltage VA, and a resistor 18 also arranged along the power supply line 14 and presenting a resistance whose value is proportional to the resistive drop of the electronic components (not shown) connected upstream of the lighting device 1.
Regarding the driving stage 12, it is configured in such a way as to make the lighting branches 4 transit from a pre-lighting state, in which the LEDs 7 are flowed through by a predetermined minimum secondary current I2=I2M dimensioned so that the light emitted by the LEDs 7 has a predetermined minimum light intensity, and a complete-lighting state, in which the LEDs 7 are flowed through by a predetermined nominal secondary current I2=I2N dimensioned so that the light emitted from the LEDs 7 has a fixed maximum intensity.
In particular, the driving stage 12 comprises a switch 19, which has a first terminal connected to the common node 6, a second terminal connected to the power supply line 15, and a control terminal receiving a logic control signal COM adapt to assume a logical high or low value to respectively control the closing or opening of the switch 19.
The driving stage 12 further comprises a logic gate AND 20 presenting a plurality of inlet terminals connected to the sensing nodes 8 of the lighting branches 4, and an outlet terminal connected to the control terminal of the switch 19.
The driving stage 12 finally comprises, a pull-down circuit 21, which in turn is provided with a series of pull-down resistors 22, each of which is connected between an inlet terminal of the logic gate AND 20 and the power supply line 15, i.e. to ground.
The operation of the lighting device 1 is known and will be resumed below only with regard to the aspects necessary for the understanding of the issues pertaining to the present invention.
In particular, in the pre-lighting state, the switch 19 is opened, the common node 6 of the array of light emitting sources 2 is disconnected from the power supply line 15 and consequently the secondary current I2 flowing through the LEDs need to discharge to ground by way of the pull-down resistors 22 which limit the same to its predetermined minimum value I2M. At this step, the LEDs 7 being flowed through by the minimum secondary current I2M, emit light with a predetermined minimum luminous intensity.
The lighting device 1 transits from the pre-lighting state to the complete-lighting state, when the voltage at the heads of the pull-down resistors 22 is brought from low logic value to high logic value. In this case, the logic gate AND 20 switches on its output terminal the control signal COM to a high logic value, causing the closing of the switch 19 and therefore determining the connection between the common node 6 and the power supply line 15.
At this stage, the secondary current I2 flowing through the LEDs 7 increases until reaching its nominal value I2N thus leading to the emission from the LEDs 7 themselves of a light with nominal intensity.
Thanks to the circuit architecture of the driving stage, the above described lighting device 1 is conveniently able to automatically transit from the complete-lighting state to the pre-lighting state whenever there is a sudden increase in impedance along a lighting branch due to a failure of a LED.
In particular, if the failure of a LED causes the opening of the corresponding lighting branch, it occurs: the interruption of the secondary current flowing through the lighting branch itself, the commutation of the voltage logic value in the corresponding sensing node 8 from top down, the commutation of the voltage logic value of the control signal COM from top down in the output terminal of the logic gate AND 20, and consequently the opening of the switch 19.
However, the circuit architecture of the driving stage 3 described above has the following technical problems.
First, the electronic control circuit of the lighting device described above is able to bring the latter in the pre-lighting state, i.e. in a state of circuital security in which the secondary current flowing through the properly working lighting branches has the minimum value, only in case of a failure corresponding to the opening of the LED, but is unable to intervene in the same way when the failure of a LED causes a short circuit between the terminals of the LED itself.
In particular, in this last case there is an uncontrolled increase of the secondary current flowing in the faulted branch which puts the lighting device in a critical electrical condition being potentially harmful to the electronic components thereof and simultaneously determines the same lighting effect completely dissimilar with respect to a possible on/off state of a traditional single-source light. In fact, at the occurrence of this faulty condition, the LEDs of the lighting branch containing the short circuited LED generate an unwanted distribution of light onto the front lenticular body.
Secondly, the circuit architecture of the electronic control circuit does not reduce the minimum value of the secondary current I2M below a limit threshold, so as to completely interrupt light emission from the LEDs in the pre-lighting state, therefore obtaining a lighting effect equivalent to that of a traditional single-source light in the off state.
In fact, such a reduction would request increasing the resistance of the pull-down resistors and/or simultaneously, a reduction in supply voltage.
However, the resistance of the pull-down resistor can be increased up to a limit beyond which the circuital constraints associated with operational logic voltages of the logic gate AND are no longer satisfied, while the supply voltage has a predetermined standard values range, typically between 8 and 16 volts.
Finally, in the case the LED has a fault corresponding to an open circuit, it occurs, during the device transition from an off state when the primary supplying current is zero, to the pre-lighting state, a temporary increase in secondary current flowing in the properly working branches, which results in the emission of an unwanted pulse of light, the latter condition, even in this case, differs from a traditional single-source light.